Geomagnetically induced currents (GICs) are a naturally-occurring phenomenon initiated by solar activity. Sunspots (i.e., relatively cool areas on the sun shielded by complex magnetic fields) can give rise to solar flares and coronal mass ejections (CMEs). A CME carries its own electromagnetic currents and magnetic fields that are capable of affecting the Earth's magnetic field. Charged particle movement in the conductive ionosphere increases the current flows in the electrojets, which are currents in the order of millions of amperes located more than 100 kilometers above the Earth's surface. Electrojet currents induce quasi-DC voltages in electrical transmission lines, which, in turn, drive the flow of GICs wherever there is a path for them to flow.
GICs are often called quasi-DC currents because of their generally low frequency (0.1 mHz to 0.1 Hz), and thus the network response is essential resistive. A geomagnetic disturbance (GMD) event can last for one to two days, and continually generate relatively low to moderate levels of GICs with several intermittent periods of high GICs. The GICs can result in nonlinear operation of a power transformer and a significant increase of the exciting current. This also may then lead to generation of harmonics in the electrical current, unnecessary relay tripping, increased reactive power demands, voltage fluctuations and drops, and even a black-out of the entire system. Transformers may be overheated and, in the worst case, permanently damaged.
Prior art methods include using a capacitor or a capacitor with a resistor and metal oxide varistor (MOV) to mitigate the effects of GICs on transformers. However, these solutions generate extremely high voltages on such elements under short circuits in the main. Further, the value of the capacitor is kept fixed, which may act as an over-control or under-control, as is the control duration and magnitude. In addition, prior art methods are expensive.
Accordingly, what is needed is a cost-effective method to mitigate the adverse effects of GICs on power transformers.